As a light emitting type electronic display device, electroluminescent displays (hereinafter abbreviated as ELD) are known. Elements constituting the ELD include an inorganic electroluminescent element (hereinafter also referred to as an inorganic EL element) and an organic electroluminescent element (hereinafter also referred to as an organic EL element). Though the inorganic EL elements have been employed as a flat type light source, a high voltage alternating current has been required to drive light emitting elements.
On the other hand, the organic electroluminescent element has a constitution in which a light emitting layer incorporating light emitting compounds is sandwiched between a cathode and an anode, and emits light utilizing release of light (fluorescence and phosphorescence) when excitons are deactivated, which excitons are generated by recombination of electrons and holes injected into the light emitting layer. The above element has attracted attention in view of the following points: it is capable of emitting light at a few to several ten volts, and further exhibits a wide viewing angle due to a self light emitting type, high visibility, and space saving and portability due to a complete thin layer type solid element.
Further, the organic electroluminescent element also exhibits a major feature that it is an area light source differing from conventionally employed main light sources such as a light emitting diode or a cold-cathode tube. Possible applications utilizing the above characteristic include light sources for lighting and backlight of various displays. In particular, it is appropriate to employ it as a backlight for liquid crystal full color displays, of which demand is markedly increasing over recent years.
When the organic electroluminescent elements are employed as the above lighting source or display backlight, they are employed as a light source emitting white or electric bulb color light (hereinafter together referred to as white).
In order to realize white light emission with the organic electroluminescent elements, there are methods such as a method which realizes white via color mixture by preparing in a single element a plurality of light emitting materials differing in their emitted light wavelength; a method which realizes white in such a manner that three colors, for example, blue, green, and red are separately coated and they are simultaneously emitted to make a mixture of the colors; and a method which realize white employing color conversion dyes (for example, a combination of a blue light emitting material and a color conversion fluorescent dye).
However, when consideration is made for various demands for the light source for lighting and backlight such as lower cost, higher productivity, and more convenient driving methods, the method which realizes white via color mixture by preparing in a single element a plurality of light emitting materials differing in emitted light wavelengths, is useful for these applications, and in recent years, research and development of the same have been increasingly conducted.
The methods which realize white based on the above method will further be detailed. The methods include a method which realizes white by employing in an element two different color light emitting materials, which are complementary colors each other, such as a blue light emitting material and a yellow light emitting material, and mixing their colors, and a method which realizes white by employing light emitting materials of three colors of blue, green, and red and mixing their colors.
For example, a method for preparing a white organic electroluminescent element is disclosed, in which doping is carried out employing three fluorescent materials of blue, green and red, exhibiting high efficiency as a light emitting material (for example, Patent documents 1 and 2).
However, above methods had problems that they not only give insufficient light emission efficiency, but also exhibit large fluctuations in light emission colors depending on electric current density.
Further, there is a system in which, in organic electroluminescent elements emitting white light, each of the layers differing in emitted light color is not in the form of an individual layer, but at least two color light emitting materials are made to coexist in a single layer, and two color lights are emitted via an energy transfer from a light emitting dopant with high light emitting energy to a light emitting dopant with a relatively low efficiency (for example, Patent documents 3 and 4).
The above method is one of the promising methods to prepare a white light emitting organic EL element, because the above method can reduce the number of organic layers and to decrease the employed amount of light emitting materials. The light emission efficiency has been increasing by the above methods, but they do not contribute to improvement in color rendition.
In the meantime, in recent years, other than the fluorescent materials, developments of phosphorescent compounds, by which organic electroluminescent elements exhibiting higher luminance can be obtained, have been energetically carried out (refer, for example, to Patent Document 4 and Non-Patent Documents 1 and 2).
The light emission from conventional phosphorescent materials is one from an excited singlet, and since a generation ratio of a singlet exciton to a triplet exciton is 1:3, a generation probability of light emitting exciton species is 25%. On the other hand, in case of phosphorescent light-emitting compounds which utilize light emission from excited triplet, since the upper limit of internal quantum efficiency becomes 100% by generation ratio of an exciton and an internal conversion of a singlet exciton to a triplet exciton, the light emission efficiency principally becomes at most 4 times compared to that of fluorescent light-emitting compounds.
However, regarding phosphorescent light-emitting compounds especially exhibiting blue light emission, there have been no compounds exhibiting excellent light emission efficiency and durability, in particular, a driving lifetime. As a result, heretofore, it has been difficult to develop practical organic electroluminescent elements employing blue phosphorescent light-emitting compounds.    Patent Document 1: Japanese Patent Application Publication (hereinafter also referred to as JP-A) No. 6-207170    Patent Document 2: JP-A No. 7-41759    Patent Document 3: JP-A No. 2003-68465    Patent Document 4: JP-A No. 2003-77674    Non-Patent Document 1: M. A. Baldo et al., Nature, Vol. 395, pages 151-154 Non-Patent Document 2: M. A. Baldo et al., Nature, Vol. 403, No. 17, pages 750-753